1. Field of the Invention
This invention pertains generally to the field of fluid flow and more particularly to the field of fluid flow control in wastewater treatment systems.
2. Description of the Prior Art
Typical wastewater treatment systems may include a a clarifier tank wherein floatable and settleable solids are removed. The settleable solids tend to sink under the force of gravity and floatables are skimmed off the top. Inflow to such a tank is usually close to the bottom, at which level the water is passed through a stilling well to spread out the flow and produce a smooth, even and slowly rising water current. Outflow from the tank is typically accomplished by means of a weir trough structure surrounding the tank or positioned inward of its sidewall.
Since water will tend to seek the most direct path in the tank between the fluid intake and the outflow weir structure, there will inevitably be some tunneling or channeling currents having a lateral component wherein the rate of flow increases. This in turn tends to produce quiescent or stagnant zones in the tank which are relatively unaffected by influent water. The result is that otherwise settleable solids may be carried within the more rapidly moving channel currents and swept out of the tank while the available dwell time of influent water within the tank is decreased.
This problem is aggravated when the tank, particularly one of large diameter, say 50 feet, experiences tilting such as may occur as the tank settles in use. For example, with V-notch weirs disposed about the periphery of such a tank, a relatively small angle of tilt, by simple geometry, produces a considerable vertical displacement between oppositely positioned V-notches and, thus, a big difference in head. This head imbalance may be great enough to create significantly stronger, non-uniform channeling currents directed toward the lower weirs. A comparable effect may result if some of the V-notch weirs become plugged with surface debris and scum. Part of the problem, of course, lies in the fact that the rate of flow through a V-notch weir is highly sensitive to changes in upstream head. The problem is compounded by the fact that misalignment becomes difficult to detect if the tank is covered. It is also difficult to determine in that event whether particular V-notches are, in fact, plugged. For very large covered tanks, inspection, servicing, and adjustment of perimeter notch-type weirs without draining the tank becomes virtually impossible.